The present invention relates to a light emitting device package and a method for manufacturing the same.
In general, a light emitting device basically comprises a junction of p-type and n-type semiconductors, and is a kind of optoelectronic device in which upon application of a voltage thereto, energy corresponding to a bandgap of a semiconductor is emitted in the form of light due to combination of electrons and holes.
The amount of light outputted from the light emitting device increases in proportion to an electric current flowing through the device.
Studies on such a light emitting device has been actively conducted as power consumption is low, harmful substances are not used, color reproducibility is good and a life span is extended as long as tens of thousands hours or more.
A light emitting device such as a light emitting diode (LED) or laser diode using a III-V group or II-VI group compound semiconductor material of a direct transition type semiconductor can implement red, green, blue colors and ultraviolet rays by means of a thin film growth technique and development of a device material.
Further, the light emitting device can implement a white light source with superior efficiency by using a fluorescent substance or by combining light.
With the development of such techniques, the light emitting device has been used not only in a device for a display but in a variety of fields such as a transmitting module, a backlight of a liquid crystal display (LCD), a lighting system capable of substituting for a fluorescent lamp or incandescent bulb, a sign board, a traffic light, and an instrument panel of a car.
Meanwhile, if a substrate is n-type in a red or infrared light emitting device, an n-type layer, an active layer and a p-type layer are sequentially laminated on a top surface of the substrate, an n-type electrode layer is formed beneath a bottom surface of the substrate, and a p-type electrode layer is formed on a top surface of the p-type layer.
If a forward voltage is applied between the n-type and p-type electrode layers in the red light emitting device, electrons are injected into the active layer through the n-type layer and holes are injected into the active layer through the p-type layer.
At this time, the electrons and the holes injected into the active layer are recombined and then emit light corresponding to a bandgap or an energy level difference of the active layer.
As described above, the red or infrared light emitting device is provided with a structure where the p-type and n-type electrode layers face each other with the substrate there between.
Accordingly, at least one electrode should be subjected to wire bonding in the prior art so that the red or infrared light emitting device can come into contact with a submount or a printed circuit board (PCB) and can be electrically connected to other devices.
Therefore, a certain space for wire bonding to the submount or the PCB should be provided. This causes increase in the size of a package, and reduction in reliability of the package due to short or disconnection of a wire.
Such a prior art will be described with reference to FIG. 1.
FIG. 1 is a sectional view of a conventional light emitting device package using wire bonding. First and second conductive pads (110, 111) are formed on a top surface of a substrate (100) while being spaced apart from each other.
Further, a second electrode layer (123) formed beneath a bottom surface of a light emitting device (120) is bonded to a top surface of the first conductive pad (110), and a first electrode layer (122) formed on a top surface of the light emitting device (120) and the second conductive pad (111) are wire bonded for electrical connection there between.
A submount made of silicone (Si) or ceramic, or a PCB is used for the substrate (100).
The first conductive pad (110) is formed on the top surface of the substrate (100).
Further, the first conductive pad (110) has the same polarity as that of the second electrode layer (123) formed beneath the bottom surface of the light emitting device (120).
For example, if the second electrode layer (123) formed beneath the bottom surface of the light emitting device (120) is n-type, the first conductive pad (110) is also n-type. If the second electrode layer (123) formed beneath the bottom surface of the light emitting device (120) is p-type, the first conductive pad (110) is also p-type.
The second conductive pad (111) is formed on the top surface of the substrate (100) such that it is spaced apart from the first conductive pad (110).
The light emitting device (120) is bonded to the top surface of the first conductive pad (110).
FIG. 2 is a sectional view showing a state where light emitting devices are arrayed and packaged according to a prior art. A plurality of conductive pads (112a, 112b, 112c, 112d, 112e, 112f) is formed on a substrate (100) of the light emitting device package.
In addition, a lower electrode (131) of a red light emitting device (130) is bonded to a top surface of the conductive pad (112a) with a conductive adhesive (135), and an upper electrode (132) of the red light emitting device (130) is wire bonded to another conductive pad (112b).
Further, a green light emitting device (140) is bonded to the substrate (100) with an adhesive (145), and two upper electrodes (141, 142) of the green light emitting device (140) are respectively wire bonded to the two conductive pads (112c, 112d).
Furthermore, a blue light emitting device (150) is bonded to the substrate (100) with an adhesive (155), and two upper electrodes (151, 152) of the blue light emitting device (150) are respectively wire bonded to the two conductive pads (112e, 112f).
Such a light emitting device package mounts red, green and blue light emitting devices on a substrate to implement a package emitting white light. At this time, wire bonding is performed several times.
As described above, there is a disadvantage in that a space for wire-bonding a light emitting device is additionally required in the conventional light emitting device, resulting in increase in the size of a package.
Further, since a bonded wire may be short-circuited or disconnected, reliability for connection between devices suffers.
Furthermore, the wire bonding is not suitable for mass production.
Meanwhile, roughly two methods can be used to fabricate a white light emitting diode. First, there is a single chip-type method in which a fluorescent substance is combined on a blue or UV LED chip to obtain white light. Second is a multi chip-type method in which two or three LED chips are combined with each other to obtain white light. At this time, if where the single chip type method is used, it is essentially required to apply a phosphor on a fabricated LED.
FIGS. 3a and 3b are sectional views schematically showing a state where a phosphor is applied to a conventional light emitting device package. An LED having a p-type layer (157), an active layer (156) and an n-type layer (155), which are sequentially laminated one above another, is joined to a submount (160) with a p-ohmic contact metal layer (162) interposed therebetween, and an n-ohmic contact metal layer (163) is formed on the n-type layer (155).
Here, a wire (159) through which a current is supplied to the LED is bonded to the n-ohmic contact metal layer (163).
At this time, a phosphor is applied to surround the LED and a portion of the wire (159) as shown in FIGS. 3a and 3b. 
Generally, in order to form a phosphor thin film, it is preferred that there be no convex or concave portion on a top surface of a device to which the phosphor is to be applied. However, since the wire is bonded to the top of the LED in the conventional light emitting device package shown in FIGS. 3a and 3b, it is not easy to apply the phosphor without damaging the wire.
Further, to perform the wire bonding, a device is fabricated considering a pattern area of a bonding pad. However, if a wire bonding portion comprising the bonding pad and the wire is placed on the top of the LED, a disadvantage occurs that it partially conceals a vertical light emitting area.
That is, although an area of about 0.1×0.1 mm2 is required for wire bonding, the wire bonding portion conceals 1/9 of a light emitting area in a chip of 0.3×0.3 mm2.
Further, tendency is that, in manufacturing a high-performance LED its entire area comes to be larger, and if necessary, the number of ohmic metal pads may be increased to reduce electrical resistance.
Since a high-performance LED is operated with a high current, it will be apparent that serial resistance should be reduced to prevent heat accumulation. Further, an ohmic contact metal is made thicker to reduce a voltage drop there by enhancing the light extraction efficiency.
However, since there is limitation on deposit of a thick metal and the area of a wire bonding pad positioned on the top of an LED should be increased to prevent deterioration of the performance of the LED due to a voltage drop within the ohmic contact metal, a vertical light emitting area of the LED inevitably decreases.